Radio frequency tank circuit



Oct. 5, 1954 c. J. STARNER ETAL RADIO FREQUENCY TANK CIRCUIT 2 Sheets-Sheet 1 Filed Sept. 27, 1951 l l L CAI/FILES INVENTORS Jayme: mp 75 000 oA/[WMl/V ATTORNEY Oct. 1954 c. J. STARNER ETAL RADIO FREQUENCY TANK CIRCUIT 2 Sheets-Sheet 2 Filed Sept. 27, 1951 I gYENTORS 35 F 3 ATTORNEY WPW 70 56 Patented Oct. 5, I954 STAT TNT OFFICE RADIO FREQUENCY TANK CIRCUIT of Delaware Application September 27, 1951, Serial No. 248,542

7 Claims.

This invention relates to radio frequency tank circuits, and more particularly to tank circuits suitable for use in high power grounded-grid triode amplifiers.

An object of the present invention is to pro vide a tank circuit for the anode of an electron discharge device, having a minimum amount of lead inductance and tunable over the range of 4 to 27 megacycles.

Another object is to provide a rotating ring tuning mechanism for a variable helical inductor having spaced turns. which is compact and capable of adjusting itself to variations in coil turn spacing and to surface irregularities.

Still another object is to provide an improved variable inductor suitable for use in the tank circuit of a high power electron discharge de- VICE.

A feature of the invention is an anode blocking condenser in the form of inner and outer U- shaped capacitor plates which surround the inductor, with the inner plate attached directly to one end of the inductor to minimize lead inductance.

Another feature of the invention is the rotating ring contacting mechanism for the variable inductor, which is supported and guided by the spaced turns of the inductor.

The invention is hereinafter described, by way of example only, in connection with a groundedgrid triode amplifier capable of providing output of large power over a relatively wide range of frequencies. The inductor is in the form of a helix with spaced turns. A rotating contact ring, which is guided and supported by metallic rollers inserted between the turns of the helix, supports a plurality of spring loaded equally (circumferentially) spaced brush contacts which make contact with the coil. The contact ring consists of two circular spaced metallic flanges or angle irons. A circular ring gear, mounted between the two flanges of the contact ring, meshes with two pinions which are slidable on two rotatable square shafts. These shafts are connected to the inner of two U-shaped capacitor plates surrounding the inductor on three sides. The bottom of the inductor is connected to the inner U-shaped capacitor plate by means of a mounting plate which supports the inductor and the inner capacitor plate. contacts, extending over substantially the entire length of the inductor and secured to the inner capacitor plate on opposite sides thereof, continuously engage the contact ring which r tates and moves lengthwise of the spring contacts and the inductor. These springs together with the contact ring act to short out a desired portion of the inductor as the anode circuit is tuned. The amplifier vacuum tube is mounted directly over the inductor and has its anode Resilient spring I connected directly to one end of the inductor. A metallic housing, which is electrically grounded, encloses the inductor and the U-shaped capacitor plate assembly and insulatingly supports the tube. Insulators connect the outer capacitor plate to the housing. The capacity between the U-shaped plates operates as a direct-currentblocking capacitance, and the capacity between the outer capacitor plate and the housing functions as the ground return capacitance, both capacitances having a minimum amount of inductance. Air is forced through a large aperture in the mounting plate past the inductor and the metallic tube to cool the entire apparatus.

An advantage of the invention is that a very short low impedance ground return path and a low inductance lead between the variable inductor and vacuum tube are provided to allow for efficient harmonic-free operation, particularly at the high frequency end of the operating range.

Other objects, features and advantages will appear in the subsequent detailed description which is accompanied by a drawing, wherein:

Fig. 1 is a front elevation, partly broken away and partly in section, showing the grounded-grid amplifier and tank circuit structure, including the inductor tuning mechanism and the capacitor plate assembly, in accordance with the invention;

Figs. 1a and lb are views drawn on an enlarged scale to illustrate details of Fig. 1;

Fig. 2 is a sectional View taken along line 2-2 of Fig. l; and

Fig. 3 is the equivalent electrical circuit of the tank circuit structure shown in Figs. 1 and 2, using the same reference numerals to indicate identical parts shown in the mechanical structure.

Referring to the figures, the inductor 4 is generally helical in configuration. The inductor is made from a heavy fiat bar having rounded edges and is mounted on a metallic mounting plate t by means of four spaced inductor support brackets 8 which are fastened to the inductor and to the plate 6. Plate 6 has a large central aperture l4 and is supported on four identical insulators Ii] arranged one at each of the four corners of said plate. The contact ring 40 which surrounds inductor A is made from two spaced metallic angle irons or flanges carrying therebetween a large ring gear 52. Each flange constitutes one-half of the contact ring.

Mounted on the inside surface of ring 40 are four metallic brackets 5d equally spaced circumferentially around the ring. Three of these brackets carry identical structure, illustrated at the right-hand side of Fig. 1, while the remaining bracket carries a slightly different structure, illustrated at the left-hand side of Fig. 1.

First referring to the structure illustrated at the right-hand side of Fig. 1, attached to the inside leg of each of the three like brackets 54 is a horizontal stud 58 which rotatably supports a respective inwardly-extending roller 56. An auxiliary metallic spring contact I, of generally U-shaped configuration, is attache'd'to each respective stud, with the base of the U positioned between the brackets inside leg an'd'the adjacent end of the respective roller. The two legs of each U-shaped contact I make contact with the edges of two adjacent turns of the inductor 4, as illustrated. Each of-the rollers 56 is inserted between two adjacent turns of the inductor, riding on the lower of such two turns so as to support and guide the contact ring 40. The diameter of each roller 56 is approximately t; inch less than the free space betweenadjacent turns of the inductonso that each roller makes contact with only the lower turn. The

rollers ride on the rounded edge of the flat bar forming the inductor 4. As illustrated, the rollers, studs, auxiliary'spring contacts "and the brackets, are all confined within the inside diameter of the contact ring'4il.

Now referring to the structure illustrated at the left-hand side of Fig. 1, attached to the inside leg of the single unlike bracket 54 are two spaced horizontal studs 3 and 5 which rotatably support the respective inwardly-extending rollers l and 9. Rollers"! and'9 are smaller in diameter than rollers 56 and are so spaced vertically as to loosely clasp therebetween a single turn of inductor 4. An auxiliary spring contact H, which may include two separate arms as illustrated, is fastened to the inside leg of this unlike bracket '54 and contacts the face of that inductor turn which is loosely clasped between rollers i and 9. The rollers 'i and 9,'since they loosely clasp therebetween a turn of the inductor, engage such turn and help to support'and guide the contact ring 40. Rollers l and 9, studs'3 and 5, contact ll and the fourth bracket 54, are all confined within the inside diameter of the contact ring 40.

To maintain the contact ring 46 horizontal, the rollers are mounted in ascending order, by means of different mounting positions on their respective brackets 54, with a difference in height between each two adjacent rollers-equal to of the (vertical) pitch'of one inductor turn. The single unlike bracket structure illustrated at the left-hand side of Fig. 1 is the leading or topmost roller structure of the four, which, as will appear hereinafter, is the structure nearest to the active turns of the inductor.

Mounted on the brackets 54 and located one'inductor turn above the turn supporting theguiding rollers 5,9 and'56' are the four'main current carrying brushes '10. Four straps 12 of flexible copper, one for each brush, are fastened to and connect the brushes id to the brackets 54 to carry the heavy currents, these straps following the movement of the brushes it. The brushes are spaced equidistantly around the'ring 49 and are individually spring-loaded by the springs 13 (see Fig. 2) to maintain close contact with the turns of the inductor and also to space the contact ring 4!) from the inductor 4. The four brushes share the flow of current, thus enabling carrying of more current fora certain temperature rise.

The brushes '10 make electrical contact with the inductor turn which is located above the turn which"supports the rollers. This -means that two adjacent turns on both sides of each roller are short-circuited at all times with the current induced in the lower turn carried by the rollers and auxiliary spring contacts. In practice, the losses which occur due to this shortcircuited turn are minor and do not interfere with the proper electrical functioning of the inductor 4. It should be noted that the brushes in carry essentially'all of the inductor current, while'the rollers on the turn below and the auxiliary spring contacts carry only the current induced in the turn by the coupling to the active turns of the inductor 4. The leading or topmost roller assembly (next to the active turns of the inductor) has the arrangement illustrated to prevent the auxiliary contact [I from carrying some of the main, heavy coil current and'thus becoming overheated. The auxiliary spring c'ontacts l and H in effect electrically-bridge the respective rollers 56 and l, B to maintain electrical contact between the rollers-and the turns of the inductor, thus avoiding sparking which would otherwise occur due to the unavoidable, intermitent mechanical engagement of the "rollers and coil turns.

Two U-shaped capacitor plates 30 and-32 surround threesides of the inductor-4. The outer capacitor plate 32 is attached to the housing 18 by eight z-shaped insulating supporters 3B -having long voltage creeping paths. The'innercapacitor plate 39 is attached toan'd supported'by the mounting plate 6 by means of the four bracket connectors 38. The capacitance between the two plates 39 and 32 functions as a direct-current-blockingcapacitor 3! as indicated inthe electrical circuit diagram of Fig.3. The capacitance between the outer plate "32 and the (grounded) housing i8 operates as the ground return capacitor 33.

Variation of the inductance of inductor 4 :is accomplished by rotating the contact ring 4ilby two pinions 42 made of an insulating material and positioned on opposite sidesof the-inductor 4. Each of the pinions-42 respectivelyslides and is driven by a corresponding one of two square non-magnetic metallic shafts' 46 havinga length equal to the axial length of the inductor-Lless two turns, approximately. Each shaftis-journaled in'a bearing member 5!) at each :end of the shaft. lhe bearing 'members'sll are attached to the inner capacitor plate 36. By rotating the shafts 46 by means of the insulator shafts fifl,

the contact ring-4E] gets a downward-or upward motion from its four rollers 56 through theintermediary of the pinions-42 whichengagethe ring gear 52. As the ring '4fl moves, it carries both pinions with it down or up along the square shafts, because a portion of ea'ch pinion extends between the two-flanges constituting the ring.

Thus, the resulting movement of the contact ring is'helical, albeit along the length of the indutcor, following the direction of winding and pitchpf the inductors turns.

Two chaimel-shapedmembersfi i, each having a length equal to'the' length of the-shafts ttpand each surrounding a corresponding shaft 45 on three sides, are attached tothe'inner capacitor plate 333. Two physically parallel spring contacts 68, of a length equal to the lengthoi the member 64, are fastened to the two open ends'of each of the channel-shaped members 84, as shown. The

spring contacts 68 make'good conta'ctwith the contact ring 40, and are stationary except for a slight elastic deflection of the .-portion in contact with-the ring 40.

The evacuated electron discharge device (vacuum tube) !2 is positioned directly over the inductor it and is supported by the insulator plate is upon which the metallic tube socket is is mounted. The metallic housing 83, which is electrically grounded, encloses the tank circuit and the tube l2. The top end of the inductor i is connected directly to the tube socket it, and thereby to the anode 2B of the tube Ii, by the strap 22. The grid terminal is of the tube I2 is connected to the housing 18 by four preferably equally spaced condensers 26, each having a low impedance to ground for radio frequencies, The filament terminals 23 of the tube l2 are coupled to the input tuner and filament energizing source 35 shown in Fig. 3. The filament circuit may be of the type disclosed in the copending application, Serial No. 248,501, filed September 27, 1951, which ripened on November 25, 1952, into Patent #2,619,6l6.

It should be noted that the inductor i, blocking capacitor 3land the tube l2, together with the housing is and the associated supported struc ture, comprise a complete radio frequency amplifier tank circuit. The blocking capacitor 3| may be considered as part of the inductor arrangement and also part of the return path from the inductor to the tube.

Referring particularly to Fig. 3, which shows very generally the electrical equivalent circuit of the mechanical construction of the invention, the anode-grid capacitance it for the anode tank circuit is, at the higher frequencies, only the interelectrode capacitance of th tube, insulator plate i i and other stray capacitances. Therefore, this capacitance is shown dotted in Fig. 3. This capacitance i2 functions in conjunction with the inductor 4, and the capacitors 2%, 3i and 33 to complete the tank circuit. It may be seen that the four capacitors BI, 33, 25 and it are connected in series across inductor i. Approximately one turn of the inductor 4 is effective at the Z'Y-megacycle end of the tuning range. it will be appreciated that a low inductance, that is, a short return path, must be provided if the circuit is to be tunable to 2'7 megacycles.

The driving energy from the exciter, not shown, is introduced at 18 to the amplifier input circuit 35. Lead it, coupled to the outer capacitor plate 32, connects the output of the amplifier to any suitable output circuit, such as a transmission line or antenna coupler. Lead '56 passes through a hole in housing 18 so as to be insulated therefrom. Anode voltage 3+ is connected to one end of the radio frequency choke H, the other end of which is connected to mounting plate is and the lower end of inductor 4.

Among the advantages of the invention are: the construction provides for short low inductance leads between the inductor and the tube and for a short low inductance return from the inductor back to the grid of the tube by means of the blocking condenser 3i, the ground return capacitor 33 and the grid capacitors 26. The use of the four brushes iii tends to minimize the inductance between the inductor 4 and the capacitor plates 3t and 32 as well as promoting a more even distribution of current to the capacitor plates as and 32. Extraneous radio frequency fields are entirely confined within the outer housing is. The entire apparatus can easily be cooled by fcrcing air through the large aperture it in the mounting plate 6, up through the hollow helical inductor Al and past the tube l2. The unique method of rotating and supporting the contact 6 ring 40 provides an unobstructed path for the cooling air.

Changing the operating frequency of the amplifier can be done simply, rapidly, and expeditiously since there is no need to change shorting straps, plug-in capacitors, neutralizing circuits or complete inductance assemblies, all of which changes are familiar to operators of the known conventional equipment. As an illustration, in one embodiment of the invention successfully tested as the tank circuit of a radio frequency amplifier providing an output of approximately 20 kw, the time required to tune the amplifier over a frequency range of 4 to 27 mega-cycles was about one minute.

New correlating the mechanical construction of Figs. 1 and 2 with the circuit of Fig one end (the upper end in Fig. l) of inductor is connected directly to the anode of tube l2 means of strap 22. The opposite end of the in-- ductor is mounted metallically on metaliic mounting plate 5, which is connected to the supply through choke ii. The movable tap on inductor s (brushes H3) is also connected met l lically and electrically to plate through per straps l2, brackets 53, the angle irons ring dd, spring contacts 88, channel members to inner capacitor plate SE3; this inner capacitor plate is attached and electrically connected to mounting plate 6 by means of metallic bracket connectors 33. Thus, that portion or inductor between brushes ii! and plate t (bottom of the inductor) is short-circuited by the metallic path. described. Inner capacitor plate 3t is connected to brushes "iii, or to the plate 6 at the lower end of the inductor, by means of the brackets 33 described. The connection it to the antenna, as described, is taken off from the outer capacitor plate 32, while housing is is grounded,

What is claimed is:

l. A radio frequency amplifier tank circuit comprising a tube having an anode, 1:1: ans to support said tube, a inductor mounted closely adjacent said tube and having one end thereof connected to said inner and outer capacitor plates surrounding a major portion of the outer surface of said-inductor, means con-- necting the inner plate to the other end said inductor, an outer grounded housing completely surrounding said plates and said tube but s ar therefrom, insulating means supporting s outer plate inside said housing, and means for coupling a load to said outer plate.

2. A radio frequency grounded-grid amplifier tank circuit comprising a grounded housing, an insulator plate attached to said housing, a tube socket attached to said insulator plate, a tube mounted in said socket and having an anode a grid, the grid of said tube being coupled to said housing and thereby to ground, an elongated inductor of generally circular outline in a crosssection taken perpendicular to the longitudinal axis thereof, said inductor being mounted closely adjacent said tube and having one end thereof connected to said anode, a mounting plate, a pin. rality of support brackets connected between the mounting plate and the other end of said induc= tor and spaced equally around said inductor, inner and outer U-shaped capacitor plates partly surrounding said inductor, the capacitance be tween the inner and outer capacitor plates providing a low impedance at radio frequency, a plurality of insulators supporting the outer capacitor plate from said housing, a plurality of bracket connectors supporting the inner capacitor plate from the mounting 'plate iandserving to electrically connect the mounting plate endrof the inductor to the inner capacitorplate, and means to vary the inductance provided by said inductor.

3. A tank circuit comprising a helical inductor having a plurality of turns spaced from each other, a capacitor, means connecting one end of the inductor to said capacitor, a rotatable ring surrounding said inductor, a guiding member mounted on said ring, the dimension of said member parallel to the axis of said inductor being smaller than the spacing between adjacent turns of said inductor, said member'being positioned to enter the space between adjacent turns of the inductor, a contact member mounted on said ring and adapted to engage'the turns of the inductor, and means for causing rotation'of said ring around said inductor and consequent movement of such ring along the length of the inductor, to thereby vary the number of turns of said inductor between said contact member and one end of the inductor.

4. A tank circuit comprising a helical inductor having a plurality of turns spaced from each other, a capacitor, means connecting one end of the inductor to said capacitor, a rotatable ring surrounding said inductor, a guiding member mounted on said ring, the dimension of said member parallel to the axis of said inductor being smaller than the spacing between adjacent turns of said inductor, said member being positioned to enter the space between adj acent'turns of the inductor, a contact member mounted on said ring and adapted to engage the turns of the 1 inductor, a plurality of resilientspring contacts engaged by said ring, means for supporting said spring contacts from a portion of said capacitor, thereby to electrically connect said spring contacts to said one end of said inductor, and means for causing rotation of said ring around said inductor and consequent movement of such ring along the length of the inductor, to thereby vary the number of turns of said inductor short-circuited by the metallic path between said contact member and said one end of said inductor.

5. A variable inductor comprising a helical inductor having a plurality of turns spaced from each other, a rotatable ring surrounding said inductor, a guiding member mounted on said ring, the dimension of said member parallel to the axis of said inductor being smaller than the spacing between adjacent turns of said inductor, said member being positioned to enter the space between adjacent turns of the inductor, said inductor thereby supporting and guiding said ring by means of said guiding member, a contact member mounted on said ring and adapted to engage the turns of the inductor, a plurality of resilient spring contacts arranged externally of said inductor, means supporting said spring contacts parallel to and along the length of the inductor, said spring contacts being engaged by said ring, means electrically connecting said spring contacts to one end of said inductor, and means for causing rotation of said ring around said inductor and consequent movement of such ring along the length of the inductor, to thereby vary the number of turns of said inductor shortcircuited by the metallic path between said contact member and said one end of saidinductor.

6. A radio frequency tank circuit comprising 8 a helical inductor having a plurality-of turns space'cl'from each otherQinnerandouter capacitor plates partly surrounding. said inductor, common means supporting said inner plate and said inductor and electrically connecting one end of said inductor to said inner plate, a rotatable ring within said inner plate and surrounding said inductor, a guiding member mounted on said ring, the dimension of said member parallel to the axis of said inductor being smaller than the spacing between adjacent turns of said inductor, said member being positioned to enter the space between adjacent turns of the inductor, said inductor thereby supporting and guiding-said ring by means of said guiding member, a plurality of contact members mounted-on and substantially equally spaced around said ring, said contact members engaging that turn of the inductor next adjacent the turn which supports the guiding member, means electrically connecting said ring to said inner .plate, and means for causing rotation of said ring. around said inductor and consequent movement of such ring along the length of the inductor, to thereby vary the number of turns of said inductor short-circuited by the metallic path between said contact members'and said one end of said inductor.

7. A radio frequency tank circuit comprising a helical inductor having a plurality of turns spaced from each other, inner and outer U- shapedcapacitor platespartly surrounding said inductor, common means supporting said inner plate and said inductor and electrically'connectingone end of said inductor to said inner plate, a rotatable ring within said inner plate and surrounding said inductor, a plurality 'of spaced guiding members mounted on the inside of said ring, the dimension of each of said members parallel to the axis of=said inductor being smaller than the spacingbetweenadjacent turns of said inductor, each of said members'being positioned to enter the space between adjacent turns'of the inductor, said inductor thereby supporting and guiding said ring by means of said guiding members, a plurality of contact memberszmounted on and substantially equally spaced aroundsaid ring, said contact members engaging that turn of the inductor next adjacent the turn which supports the guiding members, and means for causing rotation of said ring around said inductor and consequent movement of such ring along th length of the inductor, to thereby vary the number of turns of said inductor-between said contact members and one end of the inductor.

References Cited in the file of this patent UNITED STATES PATENTS l-liunber Name Date 1,980,713 Benson Nov. 13, 1934 2,066,944 Putnam Jan. 5, 1937 2,111,381 Barton Mar. 15, 1938 2,232,042 Alford Feb. 18, 1941 2,276,051 Leischner Mar. 10, 1942 2,429,085 Albin Oct. 14, 1947 2,453,489 Brontil et al Nov. 9, 1948 FOREIGN PATENTS Number Country Date 257,003 Great Britain Aug. 16, 1926 657,865 Great Britain Mar. 15, 1938 

